1. Field of the Invention
The present invention relates generally to a zoom lens and, more particularly, to a super wide angle zoom lens having a view angle of 100.degree. or more at the wide angle end and a sufficient back focal distance and suitable for use with a single-lens reflex camera of a Leica 35 mm format, an electronic still camera, a TV camera and the like.
2. Related Background Art
Proposed as a zoom lens having a large view angle were a multiplicity of conventional wide angle zoom lenses of a negative lens forefront type in which a negative lens group is disposed closest to an object, the zoom lens being composed of, e.g., negative/positive/negative/positive type four or more lens groups. However, each of a variety of the disclosed wide angle zoom lenses based on the negative/positive/negative/positive four group construction or having a greater number of lens groups has a view angle of 95.degree. or smaller. A super wide angle zoom lens having a view angle of 100.degree. or larger is not yet proposed. The wide angle zoom lenses based on, e.g., the negative/positive/negative/positive four group construction are, though already known, disclosed in Japanese Patent Laid-Open Application No. 2-201310 and U.S. Pat. No. 4,653,873. The former wide angle zoom lens disclosed in Japanese Patent Laid-Open Application No. 2-201310 has an view angle of approximately 93.degree. at the wide angle end. The wide angle zoom lens disclosed in U.S. Pat. No. 4,653,873 has a view angle of approximately 85.degree. at the wide angle end.
In the zoom lenses shown in Japanese Patent Laid-Open Application No. 2-201310 and U.S. Pat. No. 4,653,873, a first lens group is relatively complicatedly constructed and large in thickness. A positive lens positioned second from the object in the first lens group refracts an oblique ray exhibiting a large view angle greatly in the optical-axis direction. It is therefore quite difficult to make the angle much wider while the construction of the first lens group remains as it is. Supposing that the first lens group is provided with the much wider angle while the construction thereof remains it is, the negative lens closest to the object and the positive lens positioned second from the object in the first lens group augment in size. In addition, the first lens group as a whole increases in terms of thickness. Further, when obtaining a super wide angle zoom lens having a view angle of 100.degree. or greater on the basis of the construction of the first lens group described above, the positive lens disposed send from the object in the first lens group exhibits larger refractive action on the oblique ray with a wider view angle. An incident angle of the oblique ray incident on the lens positioned closer to an image than the second lens abruptly increases. The lens eventually does not transmit the beam. This problem becomes more conspicuous with a larger refracting power of the positive lens positioned second from the object in the first lens group or with a larger aerial spacing between the positive lens and a lens disposed just behind this positive lens.
On the other hand, when seeing the wide angle zoom lenses disclosed in Japanese Patent Laid-Open Application No. 2-201310 and U.S. Pat. No. 4,653,873 in terms of aberration, with widening of the view angle, a field curvature increases, an astigmatism and a distortion augment, and a chromatic aberration in a large view angle position is also deteriorated.
In particular, the zoom lens disclosed in U.S. Pat. No. 4,653,873 has a view angle of approximately 85.degree.. However, the distortion at the wide angle end, the astigmatism and the field curvature are all large. A compensation of upper chromatic aberration is insufficient and greatly produced in the plus direction. Fluctuations in the chromatic aberration which are caused on the occasion of variable power are also large. This zoom lens is not satisfactory in terms of the aberration. Hence, it is hard to further widen the view angle in terms of the aberration.
As explained above, in the wide angle zoom lenses disclosed in Japanese Patent Laid-Open Application No. 2-201310 and U.S. Pat. No. 4,653,873, it is extremely difficult to obtain the view angle of 100.degree. or larger in terms of lens construction and the aberration as well.
On the other hand, it is a common practice that focusing is effected by moving the first lens group disposed closest to the object in the zoom lens. A wide angle zoom lens adopting an internal focusing system such as an inner focus system or a rear focus system is, though already known, also disclosed in Japanese Patent Laid-Open Application No. 2-201310.
By the way, when seeing a conventionally oft-used system for extending the whole first lens group in terms of the aberration as a focusing system of the wide angle zoom lens, the fluctuations in the chromatic aberration which are caused during focusing and in the field curvature are large. Especially in the super wide angle zoom lens having a view angle of 100.degree. or greater, this phenomenon appears more conspicuously. Besides, according to the focusing system for extending the whole first lens group, the principal ray passes through a position far away from the optical axis at the short distance, and hence a front lens diameter increases. This leads to a rise in size of the entire system. As a result, unfavorably a marginal light quantity is remarkably reduced. In particular, this problem becomes more conspicuous as the view angle increases.
Therefore, as a focusing system other than the focusing system for extending the whole first lens group, a system for moving some lenses as a focusing group within the first lens group was proposed in Japanese Patent Laid-Open Application No. 2-201310 described above.
According to this focusing system proposed therein, however, it is required to secure an aerial spacing enough to move the focusing group within the first lens group. This is therefore undesirable because of bringing about scale-ups of the first lens group and the diameter of the lens closest to the object. Additionally, a relatively large lens is employed for focusing, and a weight of the moving lens group (focusing group) is thereby increased. On the occasion of auto-focusing (AF), there is caused such a defect that quick focusing is not attainable.
Further, with a movement of the focusing lens group, there remarkably varies a height of entering principal ray having the maximum view angle which is incident on the focusing lens group set inwardly of the first lens group. Hence, undesirably the fluctuations both in the field curvature and in the chromatic aberration of magnification are large. Further, in the case of widening, as in the present invention, the view angle well over 100.degree., the aberration is undesirably outstandingly deteriorated.